High tension separation is a well established technique for classifying particulate materials such as mineral ores, shredded refuse, seeds and the like. In conventional practice separation is effected according to the electrical conductivities of the constituents of the particulate mixture. Usually, particles are fed to the top of a rotating, grounded, conductive roll or drum and are bombarded with ions from a corona discharge electrode to charge the particles on the roll surface so that they will adhere to the surface of the roll. The more conductive particles tend to lose their charge rapidly by conduction and soon separate from the roll under the influence of centrifugal force and the force of gravity. Less conductive particles tend to retain their charge and remain pinned to the roll for a longer period of time so that they can be collected as a separate fraction. Generally, the charged particles are subjected to a static field which assists in drawing away the conductive particles while holding the nonconductive particles to the rotating conductive roll.
Examples of such systems are disclosed in Grave, U.S. Pat. No. 2,072,501; Hewitt, U.S. Pat. No. 2,314,940; Johnson, U.S. Pat. No. 2,687,803; Roberts, U.S. Pat. No. 2,737,348; Breakiron, U.S. Pat. No. 3,322,275; and Barthelemy, U.S. Pat. No. 3,308,948. A modified system in which the grounded conductive roll is comprised of alternating disks of conductive and nonconductive material is disclosed in Payne, U.S. Pat. No. 994,870. Another modified system in which the roll is provided with a surface layer of highly resistive or semiconductive metal oxide is described in Fraas, U.S. Pat. No. 3,012,668. All of these systems depend on differences in the conductivities of the different materials to achieve separation.
British Pat. No. 662,463 discloses treating the surfaces of a mixture of diamond particles and gangue with an electrolyte solution and then subjecting the treated ore to conventional type separation in order to remove the diamonds from the rock or sand with which they are associated based on the differences in conductivity. Also discussed is a procedure for classifying the separated diamonds in which the diamond concentrate is fed over a metallic or nonmetallic belt past an ionizing electrode and predominantly round and predominantly flat diamond fractions are caused to separate from the belt.
When attempts are made to apply conventional electrostatic or high tension techniques to the treatment of vermiculite ores in order to separate the vermiculite from its associated gangue, satisfactory results are not obtained. Conventional electrostatic techniques can sometimes be utilized to separate vermiculite from gangue at particle sizes less than about 6 mm in size, but as the particle size increases, it becomes impossible with prior art methods and equipment to effectively separate the vermiculite particles from the associated gangue. The electrical resistivity of vermiculite ranges from about 2.times.10.sup.9 to about 70.times.10.sup.9 ohms/cm while the resistivity of the stone in a typical vermiculite gangue ranges from about 150.times.10.sup.9 to about 800.times.10.sup.9 ohms/cm. Such a difference is simply too small for effective separation by conventional electrostatic techniques. Similar problems occur in separating mica from its ore.
Conventional electrostatic or high tension separations also require drying of the material being processed prior to feeding it to the separator. Ordinarily, moisture levels must be reduced to about two percent or less for conventional techniques to be effective. It will be appreciated that the need for such drying introduces undesirable expense and inconvenience into the separation process.
Although it is not possible to effectively separate vermiculite from the associated gangue utilizing conventional high tension separation techniques which depend on differences in conductivity, it has been discovered that high tension can be utilized to separate vermiculite from its gangue if the separator roll is covered with a layer of nonconducting resin. This achievement is based on the discovery that the vermiculite particles and the gangue particles in the ore tend to have different characteristic particle shapes. The vermiculite tends to take the form of comparatively flat crystalline particles. The gangue tends to have a more nearly spherical particle shape. By treating the ore with a separator having a nonconductive layer over the separator drum, it is possible to separate the vermiculite and the gangue based on their characteristic particle shapes. Moreover, drying of the ore is not necessary. Moisture contents of up to about twenty percent by weight do not prevent effective separations.